The speed status effect simply does not work in version 1.9.2 with a previous version of Java despite working fine before. Tested at x1, x15, and x20 potency, through potion, command, and command block. Other effects such as Jump Boost work fine.
With the latest Java update, the issue is fixed.
Related issues
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Yes. This was noticeable in a parkour map I made with various effects, such as Speed and Jump Boost, as well as teleporters to distant chunks.
I don't know why that would cause Speed to stop working, though.

Because then you experience MC-92916, teleporting to unloaded chunks breaks quite some things
Have you've been teleporting to unloaded chunks?